We have been working on an in vitro model to assess the biological effects of dietary fiber on colonic cell lines. Although fiber is defined as a non-starch polysaccharide that cannot be digested by man, there is a considerable amount of degradation of fiber that occurs in the gut as a result of bacterial action. The metabolites released into the colonic lumen following anaerobic fermentation by gut bacteria may elicit a significant biological response in colonic mucosal cells. We incubated wheat bran, oat bran, corn bran, rice bran, galactomannan, and pectin individually, with human fecal inoculum under anaerobic conditions. The supernatant obtained after fermentation was lyophilized, solubilized in water and passed through an 0.22 micron filter. The effect of "fermented" as well as "unfermented" fibers were studied on three colonic cell lines: CaCO-2, LS 174 and SW 480. The end points used to monitor the effects of the various fibers on these cell lines were 1) cell proliferation using the MTT assay, 2) thymidine uptake using 3H-thymidine, 3) PGE2 synthesis by radioimmunoassay, and 4) CEA production by radioimmunoassay. We have found distinct differences in the response of the three cell lines to the different fibers. Pectin and wheat bran have been most effective in evoking a rather dramatic response in the MTT as well as the PGE2 assays. Furthermore, the evidence seems to suggest that fermentation enhances the biological activity of the fibers.